It is well established that “electronic” fuel injectors use a solenoid to control a hydraulic valve. In a solenoid, a magnetic field traverses the non-ferromagnetic free space between the ferromagnetic pole surfaces. The pole surfaces are then attracted to each other by the mechanical force generated at these surfaces.
Two natural characteristics make a solenoid difficult to use for arbitrary speed rate shaping. First, it is not a proportional displacement device. Stability forces it to be either fully at one end or the other of its travel or moving toward one of these ends. Any shaping is more a result of inherent solenoid behavior rather than what would be desirable to control fuel combustion with air. Second, its mechanical force arises from the discontinuity in permeability at its pole surfaces, which limits its force capability, and this force is an inverse function of the square of the distance between the poles. Thus, the solenoid is inherently an unstable, nonlinear device anywhere between its two ends of travel. The resulting slow “bang-bang” operation increases injector complexity needed to rate shape injected fuel.
Limited force results in limited valve speed capability. U.S. Pat. No. 6,298,829 teaches that the fastest solenoid-operated valve opens in no less than seven hundred microseconds though this time may have been reduced. The need for varying the volumetric fuel flow rate during injection has been defined by SAE papers 981927, 981928, and 981930. Rate shaping for minimum production of diesel particulate matter (DPM) and oxides of nitrogen and maximum fuel economy requires much quicker and more controllable response, which in turn requires much higher and proportionally controllable mechanical force.
When considering practical ways to quickly and compactly generate high and proportionally controllable force, control of electromagnetic phenomena is first choice. In this category, there are only two classes of transducing materials around which to construct a transducer. Both material classes have much higher specific energy than can be obtained using electrostatic or electrodynamic (solenoid) means, enabling fast, compact, and relatively powerful transducers to be constructed. These solid transducing materials strain when subjected to either an electric field or a magnetic field.
The materials that respond to electric fields are known as electrostrictive, ferroelectric, and piezoelectric but will herein be referred to only as piezoelectric material or simply piezos. Similarly, the materials that respond to magnetic fields are known as magnetostrictive, ferromagnetic, and piezomagnetic but will herein be referred to only as magnetostrictive material. As a single class, these materials are referred to herein as shape change materials or SCMs. Colloquially, SCMs are also known as smart materials. There are many other types of SCMs but they are not addressed herein as they are not candidates to drive arbitrary speed fuel injectors. In particular, shape memory alloys lack speed and ferromagnetic shape memory alloys lack force.
The main features of interest combined by the magnetostrictive and piezo SCMs are their specific energy and speed. Since their 19th century discovery, a large body of specialized knowledge of their characteristics and optimum use has been developed. In contrast to surface forces such as those generated by a solenoid, SCMs produce powerful internal body forces which allow the energy coupled per unit volume to be much higher than presently possible per unit surface area of that volume. Because of this, one of their best developed high power electrical to mechanical conversion uses has been in Navy sonar sources.
Due to the high forces that SCMs exhibit, the first law of thermodynamics limits strain. The strains that can be obtained are similar in magnitude to thermal strains, but SCMs are useful because these forceful strains can be obtained much more quickly.